Method and a system for drain liquid collection and evacuation in a subsea compression system

ABSTRACT

A method for drain liquid collection and evacuation in a subsea compression system using a compressor, wherein an external drainage tank is arranged in direct flow communication with the compressor sump via a sump evacuation valve, the tank is set under compressor discharge pressure via a tank pressurizing line arranged to connect the tank to a compressor discharge flow upstream of a throttle valve, a tank evacuation line is arranged to connect the tank to the compressor discharge downstream of the throttle valve, drain liquid evacuation is accomplished through generation, by adjusting the throttle valve, of a flow restriction in the compressor discharge. The tank is set under compressor discharge pressure by opening of the pressure valve and the drain liquid flow is permitted into a reduced compressor discharge pressure downstream the throttle valve by opening an evacuation valve in the evacuation line.

BACKGROUND OF THE INVENTION

The present invention relates in general to a subsea compression systemand in particular to a method and a system arranged to accomplishdrainage of a compressor and to control collection and evacuation ofdrain liquid in the compressor which is operated for gas compressionpurposes in subsea gas production.

In the production of natural gas from subsea deposits, the gas typicallyconstitutes the major component of a multiphase fluid containing a minorfraction of liquid. Conventionally, liquid is separated from the gas tobe collected in a scrubber or separator arranged upstream of thecompressor, and returned to the gas on the discharge side of thecompressor. A liquid pump is typically required to raise the pressure inthe separated liquid sufficiently to permit injection of the liquid intothe gas which leaves the compressor at elevated pressure.

However, subsea pumps and compressors are required to run for long timeperiods without maintenance and backup equipment may be required inorder to accomplish redundancy in operation. They therefore representconsiderable acquisition and production costs. Any attempt to modify thesubsea compression system in ways that reduce the complexity in designand costs would thus be encouraged by the industry.

An attempt to avoid drain liquid pumps in a subsea compression system ispreviously disclosed in WO 2010/102905. The drain liquid relief systemdisclosed in this document comprises a drain storage tank that receivesliquid which is separated in a scrubber arranged upstream of thecompressor. Drain liquid from the compressor casing is delivered to thescrubber and further into the drain storage tank. The liquid in thestorage tank can be set under compressor discharge pressure via conduitsand valves. A Venturi tube is arranged in the compressor discharge flowto generate a pressure lower than the discharge pressure in the drainstorage tank in order to suck drain liquid from the pressurized storagetank into the compressor discharge flow. Draining is done batch-wise byproper operation of valves. Notably however, the Venturi tube isintegrally formed in the compressor discharge nozzle or fitted into thecompressor piping. During the operation of the compressor, thecompressor discharge flow passes the Venturi tube. A differentialpressure is thus constantly generated at the Venturi tube suction portin effect of the Venturi principle.

SUMMARY OF THE INVENTION

Embodiments of the present invention aim at providing an improved methodand system for drain liquid collection and evacuation in a subseacompression system wherein discharge of the drain liquid can beaccomplished without requiring separate drain liquid pumps.

Embodiments of the present invention provide a method and system fordrain liquid collection and evacuation in a simplified subseacompression system wherein drain liquid collection and evacuation isaccomplished using a separator or scrubber arranged upstream of acompressor. Embodiments of the present invention include a compressorhaving a compressor inlet, a compressor discharge and a sump forcollecting liquid from wet gas. The method comprises arranging anexternal drainage tank in direct flow communication with the compressorsump via a sump evacuation valve. Also arranged is a tank pressurizingline, by which the drainage tank can be set under compressor dischargepressure by operation of a pressure valve and a tank pressure reliefline by which the drainage tank can be set under compressor inletpressure by operation of a relief valve. A tank evacuation line isprovided, by which liquid can be evacuated from the drainage tank intothe compressor discharge by operation of an evacuation valve. The tankpressurizing line is arranged to connect the drainage tank to thecompressor discharge flow upstream of a throttle valve and the tankevacuation line is arranged to connect the drainage tank to thecompressor discharge flow downstream of the throttle valve. Theevacuation of drain liquid from the drainage tank is accomplishedthrough generation, by adjusting the throttle valve, of a flowrestriction in the compressor discharge flow. The drainage tank is setunder compressor discharge pressure by opening of the pressure valve andthe drain liquid flow is permitted into a reduced compressor dischargepressure downstream the throttle valve by opening of the evacuationvalve.

An embodiment of the present invention provides a system for drainliquid collection and evacuation in a subsea compression system using acompressor having a compressor inlet, a compressor discharge and a sumpfor collecting liquid from wet gas. The system comprises an externaldrainage tank arranged in direct flow communication with the compressorsump via a sump evacuation valve and a tank pressurizing line. Apressure valve operable for communicating compressor discharge pressureto the drainage tank and a tank pressure relief line, wherein a reliefvalve is operable for communicating compressor inlet pressure to thedrainage tank are provided. A tank evacuation line, wherein anevacuation valve is operable for evacuation of liquid from the drainagetank into the compressor discharge is also provided. The tankpressurizing line connects the drainage tank to the compressor dischargeflow upstream of a throttle valve and the tank evacuation line connectsthe drainage tank to the compressor discharge flow downstream of thethrottle valve. The throttle valve is operable for generating, a flowrestriction in the compressor discharge. The drainage tank is set undercompressor discharge pressure via an open pressure valve, and drainliquid flow into a reduced compressor discharge pressure is permittedvia an open evacuation valve.

Further details and advantages achieved by the subject method and systemwill appear from the following detailed description of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be further explained below withreference made to the accompanying schematic drawings. In the drawings:

FIG. 1 illustrates a set up and flow chart of one embodiment of thesubsea compression system.

FIG. 2 shows an alternative embodiment.

FIG. 3 shows another alternative embodiment of the subsea compressionsystem of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION

FIG. 1 shows a compressor 1 in a subsea compression system. Thecompressor 1 comprises a motor 2, a rotor axis 3, and a rotor which isjournaled for rotation inside the rotor housing 4. The compressor 1 isconfigured to process wet gas that is recovered from a hydrocarbon welland delivered to the suction side of the compressor via flow pipeline toa compressor inlet 5 via an on/off valve 6. Processed gas leaves thecompressor at elevated pressure via compressor discharge 7 on thedischarge side of the compressor.

As used herein, the expression “wet gas” refers to a multiphase fluidcontaining a mixture of hydrocarbons and non-hydrocarbons in bothgaseous and liquid states. The relation between gas and liquid may be inthe order of 9:1, i.e. the gas volume fraction in the mixed fluid maynormally amount to about 90% or more, such as 95-97% gas. However, thegas volume fraction in the mixed gas flow is typically not constant butvaries over time.

The compressor 1 is designed for pressure boosting of unprocessed wellstreams and is for this purpose equipped to handle liquid volumefractions normally in the order of about 5% and may intermittentlytolerate liquid slugs, without mechanical failure or interruption ofoperation. A flow conditioner 8 of non-complex structure may be arrangedupstream of the compressor inlet for slug suppression and homogenizationof the flow before entering the compressor 1.

Liquid which enters the compressor in the mixed flow may not completelyleave via the compressor discharge. Due to leakage between rotor androtor housing, e.g., liquid may be separated from the flow through thecompressor and accumulate in a sump 9, arranged in the lower end of thecompressor 1. In order not to overfill the sump 9 and compressor casing,a system for drain liquid collection and evacuation is installed withthe wet gas compressor 1.

In particular, a system for drain liquid collection and evacuation inthe subsea compression system using the wet-gas compressor 1 comprises adrain liquid storage tank 10, in the following referred to as drainagetank 10. The drainage tank 10 is in flow communication with thecompressor sump 9 from which liquid can be dumped to the drainage tank10 via a sump evacuation line 11 and sump evacuation valve 12. The sumpevacuation valve 12 is an on/off valve which is controlled by a valvecontrol unit 13 and a level sensor S that monitors the liquid level 14in the drainage tank 10. The valve 12 may be configured to stay normallyopen, and controlled to close intermittently upon emptying of thedrainage tank 10 as will be explained further down in the specification.

The drainage tank 10 is a pressure vessel that communicates with thesuction and discharge sides of the compressor 1. In particular, thedrainage tank 10 can be set under compressor suction pressure via avalve 15 controlling the flow through a line 16 which connects thedrainage tank 10 with the inlet pipeline or compressor inlet 5.Alternatively, the drainage tank 10 can be set under compressordischarge pressure via a valve 17 controlling the flow through a line 18which connects the drainage tank 10 with the compressor discharge flow7. Liquid can be evacuated from the drainage tank 10 via a valve 19controlling the flow through a line 20 which connects a lower end of thedrainage tank 10 with the compressor discharge flow 7. The pressure line18 connects to the compressor discharge upstream of a valve 21, whereasthe evacuation line 20 connects to the compressor discharge downstreamof the same valve 21.

The valves 12, 15, 17 and 19 are on/off valves which are controllablebetween fully open and fully closed positions. In contrast, the valve 21is a throttle valve which is adjustable to set a temporary flowrestriction to the compressor discharge flow 7.

The valves 12, 15, 17, 19 and 21 are controllable in response to adetected liquid level 14 in the drainage tank 10. The liquid level ismonitored by the sensor S from which the information is transferred tothe valve control unit 13 which shifts the valves and the drainagesystem into drain liquid evacuation mode. All valves and the valvecontrol unit may be electrically and/or hydraulically powered andsupplied from top side at surface or land, as indicated schematically inthe drawings through the open-ended dotted line which leads to the valvecontrol unit 13.

In some embodiments, in normal production, the sump evacuation valve 12is open for dumping liquid from the compressor sump 9 into the drainagetank 10. Valves 15, 17 and 19 are closed, whereas the throttle valve 21is shifted fully open.

Evacuation mode is initiated as the liquid level monitor S senses thatthe liquid in the drainage tank 10 reaches a predetermined level. Inevacuation mode the sump evacuation valve 12 is closed, whereupon valves17 and 19 are opened, and the throttle valve 21 shifted to set atemporary restriction to the compressor discharge flow. In result, thepressure in the drainage tank is elevated to compressor dischargepressure via pressure valve 17 in the pressure line 18. Since inresponse to the flow restriction set in the throttle valve 21 a reduceddischarge pressure downstream of the throttle valve is communicated tothe lower end of the drainage tank 10, the pressure difference over thedrainage tank 10 forces liquid out from the tank to be introduced in thecompressor discharge flow, via the evacuation valve 19 and theevacuation line 20.

During evacuation of drain liquid from the drainage tank 10, thecompressor 1 may be accelerated temporarily in order to maintainconstant production, also in evacuation mode. Upon return fromevacuation mode the valves 17 and 19 are first set in closed positionand the throttle valve 21 is again set to fully open position. Theelevated pressure in the drainage tank 10 is then vented to thecompressor suction side via pressure relief valve 15 and relief line 16.Next, the sump evacuation valve 12 is again opened and the relief valve15 is closed.

At start-up of a compression system with a liquid filled compressorthere may be no compressor discharge pressure available to evacuateliquid from the compressor via the compressor sump 9 and drainage tank10. For this situation an additional pressure source or gas accumulator22 may be arranged in communication with the drainage tank 10 for thepurpose of blowing the tank via a valve 23 arranged in a line 24. In thecase of a gas accumulator 22, the same may be supplied pressure from asurface or land based pressure source and the valve 23 may be controlledfrom a surface platform or other host, as indicated by arrows andopen-ended lines 25, 26 in FIG. 1. Obviously, in order to avoid pressureloss, the gas accumulator 22 should be located subsea in close vicinityto the compression system and the drainage tank 10. In fact, a gasaccumulator for blowing the drainage tank at start-up of the compressionsystem may alternatively be mobile, such as carried on a remotelyoperated underwater vehicle (ROV), for e.g.

The subsea compression system of FIG. 2 differs from the embodimentshown in FIG. 1 with respect to the implementation of first and seconddrainage tanks 10 a and 10 b, connectable in series to the compressorsump 9. More precisely, a complementary drainage tank 10 a isinterconnected between the compressor sump 9 and the evacuated drainagetank 10 b. During production mode liquid is dumped from the compressorsump 9 into the first drainage tank 10 a via on/off valve 12 a, andfurther from the first drainage tank 10 a into the second drainage tank10 b via complementary on/off valve 12 b. The second drainage tank 10 bis installed in the system similarly to the drainage tank 10 of theprevious embodiment. Thus, in evacuation mode the valve 12 b is closed,the second drainage tank 10 b pressurized via pressure valve 17 andpressure line 18 and liquid is evacuated from the second drainage tank10 b into the compressor discharge 7. During evacuation of liquid fromthe second drainage tank 10 b, liquid may continuously be collected inthe first drainage tank 10 a via open valve 12 a dumping liquid from thecompressor sump 9. Each drainage tank may be associated with a separateliquid level sensor S1 and S2 which transfers the information to a valvecontrol unit 13. Each drainage tank, 10 a and 10 b, is individuallyconnectable to the compressor suction side via separate relief valves 15a, 15 b. Similar to the embodiment of FIG. 1, the second drainage tank10 b may be associated with a supplementary pressure source or gasaccumulator 22 to aid in start-up of the compressor 1 with aliquid-filled compressor housing.

The subsea compression system of FIG. 3 differs from the aforementionedembodiments with respect to the implementation of first and seconddrainage tanks 10 a and 10 b connectable in parallel to the compressorsump 9. In particular, each drainage tank, 10 a and 10 b, communicatesindividually with the compressor sump via separate sump evacuationvalves 12 a and 12 b. In similar way, each drainage tank is individuallypressurized via separate pressure valves, 17 a and 17 b, in pressurelines, 18 a, and 18 b, respectively. Each drainage tank is likewiseindividually evacuated via separate evacuation valves 19 a and 19 b,arranged in the associated evacuation lines 20 a and 20 b, respectively.Similar to the second embodiment, each drainage tank, 10 a and 10 b, isindividually connectable to the compressor suction side via separaterelief valves, 15 a and 15 b, arranged in separate relief lines 16 a and16 b, respectively. In the embodiment shown in FIG. 3, the liquid levelin each of the first and second drainage tanks arranged in parallelneeds to be individually monitored and detected, to which purposeseparate sensors, S1 and S2 are arranged to transfer the information toa common valve control unit 13. Further in the embodiment shown in FIG.3, one or two supplementary pressure sources or gas accumulators (notshown) may be arranged as disclosed above to serve both drainage tanksin common or each drainage tank individually upon start-up of thecompressor 1 with a liquid-filled compressor housing.

In the subsea compression system to which the embodiments of theinvention may be applied, liquid in wet gas supplied to the compressoris collected in a separate drain liquid tank which communicates directlywith the compressor's sump, avoiding complex tubing and valves.

The application of a controllable and adjustable throttle valve in thecompressor discharge permits drain liquid evacuation to be performedintermittently during production by generation, temporarily, of apressure drop over the throttle valve while the drainage tank isisolated from the compressor sump. The drainage tank is hereby, moreparticularly, set under compressor discharge pressure from upstream ofthe adjustable throttle valve.

In particular, a drain liquid evacuation sequence during productioncomprises the steps of: closing the sump evacuation valve; opening thetank pressure and evacuation valves; adjusting the throttle valve togenerate a pressure drop, and after the accomplishment of drain liquidevacuation: closing the pressure and evacuation valves; opening thethrottle valve, the relief valve and the sump evacuation valve, followedby closing the relief valve.

By adjusting the throttle valve a pressure difference, typically in theorder of 1 bar and above depending on operational conditions, isgenerated between inlet and outlet sides of the drainage tank, duringthe evacuation thereof.

At start-up of a subsea wet gas compressor, large volumes of liquid mayhave accumulated in the compressor housing. Since in this mode ofoperation there is no compressor generated gas pressure available todrive out the liquid, the aspects of the present invention foresees thatduring start-up of the system, liquid evacuation is performed bytemporarily pressurizing the drainage tank from a separate pressuresource or gas accumulator.

Alternative embodiments of the invention comprises first and seconddrainage tanks arranged in parallel, each drainage tank individuallyconnected to the compressor sump, the compressor inlet and thecompressor discharge flow via separate lines and valves, respectively. Amethod for operation of the system comprises: alternately evacuatingliquid from one of the first and second drainage tanks into thecompressor discharge flow while the other drainage tank is operative forcollecting liquid from the compressor sump.

Another alternative embodiment of the invention comprises first andsecond drainage tanks arranged in series one after the other in the flowdirection from the compressor sump, the first drainage tank emptyinginto the second drainage tank via an on/off tank isolation valve. Amethod for operation of the system comprises: evacuating liquid from thesecond drainage tank while the same is isolated from the first drainagetank.

A separate pressure source or gas accumulator is more particularlyarranged at or in close vicinity to the subsea compression system andconnectable to the drainage tank via suitable tubing and valves. In amodification of the system, the pressure source may be arranged at landor surface, or arranged subsea and supplied pressure from a land orsurface based pressure generator. Another modification of the systemforesees first and second drainage tanks connectable in parallel or inseries to the compressor sump.

It will be understood that in both embodiments shown in FIGS. 2 and 3,evacuation of drain liquid from one drainage tank is possible whilesimultaneously collecting drain liquid from the compressor sump in theother drainage tank.

It will also be understood that according to the drain liquid collectionand evacuation method and system taught and disclosed herein, thecompressor discharge is only temporarily manipulated during evacuation,whereas between evacuation modes, the discharge flow is unaffected andthe compressor capacity fully reserved for processing the gas throughthe compressor.

The invention is of course not in any way restricted to the embodimentsdescribed above. On the contrary, many possibilities to modificationsthereof will be apparent to a person with ordinary skill in the artwithout departing from the basic idea of the invention such as definedin the appended claims.

The written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any device orsystem and performing the incorporated method. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencefrom the literal language of the claims.

What is claimed is:
 1. A method for drain liquid collection andevacuation in a subsea compression system using a compressor, whereinthe compressor comprises a compressor inlet, a compressor discharge anda compressor sump for collecting liquid from wet gas, the methodcomprising: arranging an external drainage tank in direct flowcommunication with the compressor sump via a sump evacuation valve;arranging a tank pressurizing line by which the external drainage tankis set under a compressor discharge pressure by operation of a pressurevalve; arranging a tank pressure relief line by which the externaldrainage tank is set under a compressor inlet pressure by operation of arelief valve; arranging a tank evacuation line by which liquid isevacuated from the external drainage tank into the compressor dischargeby operation of an evacuation valve; arranging the tank pressurizingline to connect the external drainage tank to the compressor dischargeupstream of a throttle valve; and arranging the tank evacuation line toconnect the external drainage tank to the compressor dischargedownstream of the throttle valve, wherein evacuation of drain liquidfrom the external drainage tank is accomplished through generation of aflow restriction in the compressor discharge by adjusting the throttlevalve, setting the external drainage tank under the compressor dischargepressure by opening the pressure valve, and permitting the drain liquidto flow into a reduced compressor discharge pressure downstream thethrottle valve by opening the evacuation valve.
 2. The method of claim1, wherein the evacuation of the drain liquid is performedintermittently during production by generation, temporarily, of apressure drop over the throttle valve while the drainage tank isisolated from the compressor sump.
 3. The method of claim 2, wherein adrainage sequence during production comprises the steps of: closing thesump evacuation valve; opening the pressure valve and the evacuationvalve; adjusting the throttle valve to generate a pressure drop in thecompressor discharge, and after the accomplishment of the evacuation ofthe drain liquid: closing the pressure valve and the evacuation valve;opening the throttle valve, the relief valve, and the sump evacuationvalve; and closing the relief valve.
 4. The method of claim 1, whereinby adjusting the throttle valve a pressure difference is generatedbetween inlet and outlet sides of the external drainage tank, during theevacuation of the drain liquid.
 5. The method of claim 1, wherein theevacuation of the drain liquid is performed during start-up of thesubsea compression system by temporarily pressurizing the externaldrainage tank from a separate pressure source or a gas accumulator. 6.The method of claim 1, wherein a first drainage tank and a seconddrainage tank are arranged in parallel, each of the first and the seconddrainage tanks individually connected to the compressor sump, thecompressor inlet, and the compressor discharge via separate lines andvalves, respectively, and the evacuation of the drain liquid comprises:alternately evacuating liquid from one of the first and the seconddrainage tanks into the compressor discharge while the other of thefirst and the second drainage tanks is configured to collect liquid fromthe compressor sump.
 7. The method of claim 6, wherein the first and thesecond drainage tanks are arranged in series one after the other in aflow direction from the compressor sump, the first drainage tankemptying into the second drainage tank via an on/off tank isolationvalve, and the evacuation of the drain liquid comprises: evacuatingliquid from the second drainage tank while being isolated from the firstdrainage tank.
 8. A subsea compression system using a compressor,wherein the compressor comprises a compressor inlet, a compressordischarge, and a compressor sump for collecting liquid from wet gas, thesubsea compression system comprising: an external drainage tank arrangedin direct flow communication with the compressor sump via a sumpevacuation valve; a tank pressurizing line comprising a pressure valveconfigured to set the drainage tank under a compressor dischargepressure; a tank pressure relief line comprising a relief valveconfigured to set the drainage tank under a compressor inlet pressure;and a tank evacuation line comprising an evacuation valve configured toevacuate liquid from the external drainage tank into the compressordischarge, wherein the tank pressurizing line connects the externaldrainage tank to the compressor discharge upstream of a throttle valve,the tank evacuation line connects the external drainage tank to thecompressor discharge downstream of the throttle valve, the throttlevalve is configured to generate, by adjustment of the throttle valve, aflow restriction in the compressor discharge while the external drainagetank is set under the compressor discharge pressure via an open pressurevalve, and the liquid is permitted to flow into a reduced compressordischarge pressure via an open evacuation valve.
 9. The subseacompression system of claim 8, further comprising a separate pressuresource or a gas accumulator connectable to the external drainage tank.10. The subsea compression system of claim 8, further comprising a firstdrainage tank and a second drainage tank, wherein the first drainagetank and the second drainage tank are connectable in parallel or inseries to the compressor sump.